Debt - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 15, 2019	Dec. 31, 2019	Mar. 14, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Principal debt balance		$ 4,313		$ 5,281
Unamortized discount		(16)		(25)
Debt issuance costs		(17)		(25)
Terminated swaps		1		2
Current swaps		1		(3)
Less: current maturities		(1,049)		(961)
Total Long-term Debt		3,233		4,269
2020		1,051
2021		1,062
2022		300
2023		1,000
2024		300
Thereafter		600
Long-term debt maturities, subsequent year by quarter, first		0
Long-term debt maturities, subsequent year by quarter, second		313
Long-term debt maturities, subsequent year by quarter, third		738
Long-term debt maturities, subsequent year by quarter, fourth		0
Senior Notes due 2019 2.75%
Debt Instrument [Line Items]
Principal debt balance		0		406
Senior Notes due 2019 5.63%
Debt Instrument [Line Items]
Principal debt balance		$ 0		554
Senior Notes due 2020 2.80%
Debt Instrument [Line Items]
Stated Rate (percent)		2.80%
Debt, weighted average interest rate (percentage)		2.50%
Principal debt balance		$ 313		313
Senior Notes due 2020 3.50%
Debt Instrument [Line Items]
Stated Rate (percent)		3.50%
Debt, weighted average interest rate (percentage)		3.47%
Principal debt balance		$ 362		362
Senior Notes due 2020 2.75%
Debt Instrument [Line Items]
Stated Rate (percent)		2.75%
Debt, weighted average interest rate (percentage)		2.67%
Principal debt balance		$ 376		375
Senior Notes due 2021 4.50%
Debt Instrument [Line Items]
Stated Rate (percent)		4.50%
Debt, weighted average interest rate (percentage)		4.54%
Principal debt balance		$ 1,062		1,062
Senior Notes due 2022 4.07%
Debt Instrument [Line Items]
Stated Rate (percent)		4.07%
Debt, weighted average interest rate (percentage)		4.07%
Principal debt balance		$ 300		300
Senior Notes due 2023 4.13%
Debt Instrument [Line Items]
Stated Rate (percent)	4.125%	4.13%	3.625%
Debt, weighted average interest rate (percentage)		3.68%
Principal debt balance		$ 1,000		1,000
Increase to coupon rate	0.50%
Senior Notes due 2024 3.80%
Debt Instrument [Line Items]
Stated Rate (percent)		3.80%
Debt, weighted average interest rate (percentage)		3.84%
Principal debt balance		$ 300		300
Senior Notes due 2035 4.80%
Debt Instrument [Line Items]
Stated Rate (percent)		4.80%
Debt, weighted average interest rate (percentage)		4.84%
Principal debt balance		$ 250		250
Senior Notes due 2039 6.75%
Debt Instrument [Line Items]
Stated Rate (percent)		6.75%
Debt, weighted average interest rate (percentage)		6.78%
Principal debt balance		$ 350		350
Long-term Debt
Debt Instrument [Line Items]
Principal debt balance		$ 4,313		5,272
Capital Lease Obligations
Debt Instrument [Line Items]
Principal debt balance				$ 9